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Why are in-addr.arpa addresses backwards?

© December 2004 Tony Lawrence
December 2004

An "in-addr.arpa address" is a reverse DNS record, stored in a strange format. If we are considering ip, then "" is the reverse DNS record. It's used when you want to find out the host name of something you have an ip address for (for example, "dig -x " will give you that). But why is it stored backwards?

Well, if you wanted to get really geeky about it, it's the ip addresses that you are used to seeing that are backwards. Think about it this way: when you see mail.xyz.com, what's the least specific part of that? It's the ".com", because there are millions of ".com" machines. The "xyz.com" may have multiple machines, but "mail.zyz.com" nails it down to one.

Well, maybe more than one, because of round-robin DNS, or multiple MX records, but that's not important for this discussion.

Now look at the ip address - let's pretend that it's What's the least specific part of that? The "192", of course. So in names, the least specific part comes last, but in IP addresses, it comes first. Therefore, from a geek point of view, at least one of them is "backwards".

When DNS is used to find something on the internet, it always starts at the least specific. We ask ".com" to tell us who is responsible for "xyz.com" and we ask that DNS server where to find "mail". Likewise, if we are working from an address to find a name, we want to work the same way: go to the least specific first. Ask the machine responsible for 192 addresses to tell us where 192.168 is, etc. (except, of course, that 192.168 is one of the private address ranges so that doesn't really work) That's why in-addr.arpa records are stored in the same way as the names are: most specific to least.

Of course if the convention for addresses had been to present them like we present names, then you'd say " This machine is", and the in-addr.arpa address might be what we are used to seeing..

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-> Why are in-addr.arpa addresses backwards?

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More Articles by © Tony Lawrence

Fri Feb 25 15:11:30 2005: 72   anonymous

Very very well explaned thank you ever so much.


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